In fiber-optic communication, a single-mode optical fiber (SMF) is an optical fiber designed to carry light directly down the fiber, in a transverse mode. Modes are the possible solutions of the Helmholtz equation for waves, which is obtained by combining Maxwell's equations and the boundary conditions. The modes define the way the wave travels through space, characterizing how the wave is distributed in space. Waves can have the same mode but have different frequencies. This is the case in SMFs, where a SMF can have waves with different frequencies, but of the same mode, indicating that that waves are distributed in space in the same way.
A multi-mode optical fiber (MMF) is a type of optical fiber mostly used for communication over short distances, such as within a building or on a campus. Typical multi-mode links have data rates of 10 Megabits per second (Mbps) to 10 Gigabits per second (Gbps) over link lengths of up to about 600 meters (2000 feet). Multi-mode fibers have a fairly large core diameter that enables multiple light modes to be propagated.